A control assembly of this type is described in document EP-A-0,629,937. In this prior-art control assembly, the tubular member contains a subassembly which can be moved longitudinally by turning the tubular control member by means of appropriate cutouts forming cams, and this movement in turn controls the axial movement of the isolation valve. This structure is relatively complex and therefore expensive.
Furthermore, the valve for discharging the remaining compressed gas, which valve is arranged on the side of this structure, can have only a small diameter. This therefore means that it has to be actuated by a substantial variation in pressure (2 to 3.times.10.sup.5 Pa), which means that this valve has to be set laboriously. Its cost of manufacture is also relatively high on account of the number of components and machining operations required. In this prior-art control assembly too, the passage holes in the disc of the flow rate selector are longitudinally offset from the low-pressure gas outlet and have to be produced by a chemical means on account of their very small diameter (of the order of 0.1 mm for the first hole), these chemical boring means being very expensive. Finally, this technology requires extremely tight piercing tolerances on these holes (of the order of 1 micron).